A foamed molded article formed from expanded olefin resin beads is excellent in impact resistance, bending deflection, and restoring property to repetitive stress distortion in comparison to a foamed molded article formed from expanded styrene resin beads, and thus is used as a cushioning material or a packaging material for a precision component, a weight product or the like. In addition, a foamed molded article formed from expanded olefin resin beads is also excellent in heat resistance and grease resistance, and thus is also used as an automobile member such as a shock absorber, a bumper and a floor spacer. As described above, a foamed molded article formed from expanded olefin resin beads is widely used for various uses.
A foamed molded article formed from expanded olefin resin beads has been prepared as described below.
Specifically, first, olefin resin beads are put into a closed pressure vessel along with an aqueous medium, further a physical blowing agent is pressed into the vessel, and blowing agent is impregnated into the olefin resin beads under a condition of high temperature and high pressure. Subsequently, obtained expandable olefin resin beads are discharged to a low pressure region along with the aqueous medium, and foamed and expanded, whereby to give the expanded olefin resin beads. Next, the expanded olefin resin beads are molded in a mold, whereby to give a foamed molded article formed from expanded olefin resin beads.
A foamed molded article formed from expanded styrene resin beads can be prepared in the same manner as those for the foamed molded article formed from the expanded olefin resin beads described above in a point that both are obtained by molding expanded beads in a mold. However, in the process for preparing a foamed molded article formed from expanded styrene resin beads, expandable styrene resin beads containing a blowing agent may be stored for a long period of time, upon removed from a closed pressure vessel, with maintaining a foam property, and which is greatly different from the process for preparing a foamed molded article formed from expanded olefin resin beads.
Specifically, in manufacture of a foamed molded article formed from expanded styrene resin beads, first, a physical blowing agent, which is pressed into a closed pressure vessel, is impregnated into a styrene resin during polymerization or after polymerization of the styrene resin beads in the closed pressure vessel. The obtained expandable styrene resin beads can maintain a state where foam property is maintained for some period even when taken out from the inside of the closed pressure vessel, and thus can be stored or transported by a well-known method. Then, the expandable styrene resin beads are timely put into an expanding machine, and foamed and expanded by being heated in a heating medium whereby to give expanded styrene resin beads. The obtained expanded styrene resin beads are molded in a mold as described above, whereby to give a foamed molded article formed from expanded styrene resin beads.
On the other hand, an olefin resin has a property of easily permeating a physical blowing agent such as butane. Therefore, even though expandable olefin resin beads are manufactured by impregnating olefin resin beads with a blowing agent, the blowing agent dissipates after several hours of the preparation, and foam property prominently decreases. Accordingly, it is difficult to store expandable olefin resin beads for a long time after the preparation, and it is necessary to obtain expanded beads by foaming and expanding the expandable olefin resin beads within a short time after the preparation. Consequently, it is necessary to install an expanding machine and a molding machine in the vicinity of a facility for impregnation of a blowing agent, or, it is necessary to prepare expanded beads by so-called direct expansion, and thus there is a limit in a production base.
As described above, in a process of preparing a foamed molded article formed from expanded olefin resin beads, transport and storage in a state of expandable resin beads are difficult, and the transport and storage must be performed in a state of expanded beads or a molded article. Therefore, a foamed molded article formed from expanded olefin resin beads has high transport and storage cost for its preparation, and is economically disadvantageous in comparison with a foamed molded article formed from expanded styrene resin beads.
In addition, a foamed molded article formed from expanded styrene resin beads is excellent in compression strength in comparison to a foamed molded article formed from expanded olefin resin beads, and thus can increase the expansion ratio depending on its use in comparison to a foamed molded article formed from expanded olefin resin beads. Therefore, a foamed molded article formed from expanded styrene resin beads was advantageous in a point of lightweight properties in comparison to a foamed molded article formed from expanded olefin resin beads.
In order to solve the problems related to a foamed molded article formed from expanded olefin resin beads described above, techniques described below are developed, for example.
Specifically, there is developed, for example, a method of adding a cross-linking agent to a resin granular substance, which is composed of urea, polyamide, and an aliphatic olefin polymer such as polyethylene, and performing a cross-linking treatment by suspending and heating in a dispersion medium selected from water, alcohol and ketones having a low boiling point, so that the resin granular substance, which has been subjected to the cross-linking treatment, is impregnated with the dispersion medium acting as a blowing agent (refer to Patent Document 1).
In addition, there is developed a method of obtaining modified polyethylene resin beads by impregnating polyethylene resin nuclear particles with vinyl aromatic monomers, and carrying out polymerization and cross-linking of the vinyl aromatic monomers (refer to Patent Documents 2 and 3).
In addition, there is developed a method of preparing expandable modified resin beads by preparing polyethylene mixed resin nuclear particles from an acrylonitrile-styrene copolymer and a polyethylene polymer, suspending the polyethylene mixed resin nuclear particles in an aqueous medium, adding styrene monomers to this suspension and carrying out polymerization and impregnation of the blowing agent (refer to Patent Document 4).